In optical disk technologies, data can be read out from a rotating optical disk by irradiating the disk with a relatively weak light beam with a constant intensity, and detecting the light that has been modulated by, and reflected from, the optical disk.
On a read-only optical disk, information is already stored as pits that are arranged spirally during the manufacturing process of the optical disk. On the other hand, on a rewritable optical disk, a recording material film, from/on which data can be read and written optically, is deposited by an evaporation process, for example, on the surface of a substrate on which tracks with spiral lands or grooves are arranged. In writing data on such a rewritable optical disk, data is written there by irradiating the optical disk with a light beam, of which the optical power has been changed according to the data to be written, and locally changing the property of the recording material film.
It should be noted that the depth of the pits, the depth of the tracks, and the thickness of the recording material film are all smaller than the thickness of the optical disk substrate. For that reason, those portions of the optical disk, where data is stored, define a two-dimensional plane, which is sometimes called a “storage plane” or an “information plane”. However, considering that such an “information storage plane” has a physical dimension in the depth direction, too, the term “storage plane (or information plane)” will be replaced herein by another term “information storage layer”. Every optical disk has at least one such information storage layer. Optionally, a single information storage layer may actually include a plurality of layers such as a phase-change material layer and a reflective layer.
To read data that is stored on an optical disk or to write data on a recordable optical disk, the light beam always needs to maintain a predetermined converging state on a target track on an information storage layer. For that purpose, a “focus control” and a “tracking control” are required. The “focus control” means controlling the position of an objective lens perpendicularly to the information storage layer (which direction will be referred to herein as a “substrate depth direction”) such that the focus position (or converging point) of the light beam is always located on the information storage layer. On the other hand, the “tracking control” means controlling the position of the objective lens along the radius of a given optical disk (which direction will be referred to herein as a “disk radial direction”) such that the light beam spot is always located right on a target track.
Various types of optical disks such as DVD (digital versatile disc)-ROM, DVD-RAM, DVD-RW, DVD-R, DVD+RW and DVD+R have become more and more popular these days as storage media on which a huge amount of information can be stored at a high density. Among other things, CDs (compact discs) are still popular now. Currently, next-generation optical disks, including Blu-ray disc (BD) and HD-DVD, which can store an even greater amount of information at a much higher density, are under development, and some of them have already been put on the market.
To increase the storage density of an optical disk, the light beam that has been converged on the data plane of an optical disk preferably has as small a spot size as possible. The spot size of a light beam is inversely proportional to the numerical aperture NA of an objective lens for use to converge the light beam. Thus, by increasing the numerical aperture NA of the objective lens, the spot size of the light beam can be decreased.
The numerical aperture NA of an objective lens is inversely proportional to the focal length of the objective lens. That is why in an optical disk drive that uses an objective lens with a high numerical aperture NA, the distance from the objective lens to a given optical disk (which will be referred to herein as a “working distance”) becomes very short. A DVD player (with an NA of 0.6) usually has a working distance of 0.6 mm to 0.8 mm, while a dedicated BD player (with an NA of 0.8 or more) normally has a working distance of 0.1 mm to 0.3 mm.
If the working distance decreases as the numerical aperture NA increases in this manner, the objective lens is more and more likely to collide against the optical disk, which is a problem. Thus, the distance between the objective lens and the optical disk should be kept within a predetermined range by avoiding such “collision”. While the focus control is ON, the position of the objective lens is controlled such that the focal point (converging point) of the light beam is always located on the information storage layer as described above. That is why such collision rarely occurs. On the other hand, if the servo loop for establishing a focus control has failed to work for some reason during a read or write operation (i.e., in case of defocusing), then retraction processing is carried out immediately to retract the objective lens as far away from the optical disk as possible. However, before the focus control is resumed, the objective lens may still collide against the optical disk. Hereinafter, this point will be described.
Portion (a) of FIG. 1 schematically illustrates how the gap between the surface 100a of an optical disk 100 and an objective lens 22 decreases gradually. This optical disk 100 includes a substrate 112, which is transparent to a laser beam, an information storage layer 114 that has been formed on the substrate 112, and a protective layer (coating layer) 116 that covers the information storage layer 114. The optical disk 100 illustrated in portion (a) of FIG. 1 corresponds to a BD and the coating layer 16 has a thickness of about 0.1 mm.
Portion (a) of FIG. 1 illustrates a situation where the focal point of the laser beam 21 is located on the surface 100a of the optical disk, a situation where the focal point of the laser beam 21 is located on the information storage layer 114, and a situation where the focal point of the laser beam 21 is located inside the substrate 112. Portion (b) of FIG. 1 schematically shows a focus error (FE) signal to be generated when the focal point of the laser beam 21 varies with time. The focus error signal changes so as to draw a small S-curve when the focal point of the laser beam 21 passes the surface 100a of the optical disk. Portion (c) of FIG. 1 schematically shows the amplitude of a radio frequency (RF) read signal to be generated when the focal point of the laser beam 21 varies with time. When the focal point of the laser beam 21 passes the information storage layer 114 of the optical disk 100, the amplitude of the read signal shows a non-zero significant value. That is why when the amplitudes of the read signal and the focus error signal are both equal to or greater than their predetermined levels, it can be determined that the focal point of the laser beam 21 is now located in the vicinity of the information storage layer 114. If the focus servo is turned ON in such a situation, the position of the objective lens 22 is controlled such that the focus error signal is always equal to zero. Such an operation of turning the focus servo ON around the center of the S-curve of the focus error signal (i.e., near the zero-cross point of the focus error signal) when the S-curve is detected while the objective lens 22 is being moved toward the optical disk 100 in search of the information storage layer 114 will be referred to herein as a “focus finding operation”.
The S-curve appears in a relatively narrow detection range (of several μm) of the focus error signal. For that reason, to get the focus finding operation done, the operation of catching the target information storage layer 114 into the detection range by shifting the focus position of the objective lens 22 sufficiently close to the information storage layer 114 of the optical disk 100 needs to be carried out. Such an operation of moving the objective lens 22 from a position that is far away from the optical disk 100 toward the optical disk 100 gradually in order to detect the S-curve is sometimes called a “focus search operation”. The position of the objective lens 22 along the optical axis is adjusted by a lens actuator in the optical pickup. Thus, during the focus search operation, the objective lens 22 is moved toward the optical disk 100 by gradually increasing the drive current to be supplied to the lens actuator. And when the S-curve is detected in the focus error signal in the meantime, it can be determined that the target information storage layer 114 has now entered the detection range. Then, the servo operation is started and the amount of the drive current supplied to the focus actuator is controlled such that the S-curve of the focus error signal becomes equal to zero. Such a series of operations to be performed to establish the focus servo loop is sometimes called “focus ON processing”.
During the focus ON processing, if the objective lens 22 is moved too fast along the optical axis until the S-curve appears in the focus error signal, then the focal point of the light beam 21 will pass the information storage layer 114 of the optical disk 100 so shortly that the objective lens 22 may come even closer to the optical disk 100 without detecting the S-curve properly. In that case, the objective lens 22 will collide against the optical disk 100. Such a problem can be solved to a certain degree by decreasing the moving velocity of the objective lens 22. However, if there is some scratch or dust on the optical disk 100, the S-curve may not be detected properly.
Patent Document No. 1 discloses a method of preventing the objective lens from colliding against the optical disk even if detection of the information storage layer has failed and the focus control cannot be started appropriately during the focus ON processing. Specifically, the optical disk drive disclosed in Patent Document No. 1 memorizes the drive voltage of the actuator and turns the focus drive voltage OFF when the drive voltage of the actuator exceeds a predetermined level during the focus ON processing. By adopting such a technique, even if the focus search has failed due to the presence of dust or scratch on the surface of the optical disk, the focus search is stopped before the objective lens collides against the optical disk.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 11-120599        